The invention relates to a device for scanning a rotating information carrier comprising
rotating means for rotating the information carrier around an axis,
a transducer for converting a write signal into a detectable pattern at the information carrier, and/or for detecting a pattern at the information carrier and generating a read signal in response thereto,
control means for controlling a scanning parameter of the transducer, wherein the control means comprise
difference signal generating means for generating a difference signal which is indicative of a difference between a current value and a desired value of the scanning parameter,
correction signal generating means for generating a correction signal in response to the difference signal, the correction signal generating means comprising a delay loop which includes combination means for generating a combination signal out of a feedback signal and an input signal derived from the difference signal, first in first out memory means for generating a delayed signal out of the combined signal and feedback means for generating a feedback signal out of the delayed signal.
In a device for scanning a rotating information carrier a periodic error may occur in a position of a transducer due to an unbalanced or an eccentric information carrier. Likewise periodic errors may occur in other parameters, for example the accuracy with which a beam is focussed in an optical player. It has been found that such a periodic error may be substantially reduced by means of a controller comprising a memory loop. Such a controller has a relatively high gain for signals having a frequency corresponding to a basis frequency or the harmonics thereof. A substantial reduction of the periodic errors is achieved if the basis frequency of the memory loop corresponds to the rotation frequency of the information carrier. Such a controller is known from the patent application WO 96/13898. The known controller is designed such that its transfer function has a zero slope around the basis frequency and its harmonics, so that it is relatively insensitive to variations in the rotation frequency. However the controller still is only operable within a frequency-interval around the basis frequency. This is in particular a disadvantage in a device in which the information carrier is not rotated at a constant angular velocity, such as in CLV or in quasi-CLV systems. The frequency intervals within which the gain of the loop is relatively high can not be widened unlimitedly, because that would entail the risk of instabilities in the servo loop.
It is a purpose of the invention to provide a device for scanning a rotating information carrier, having a control system for controlling a scanning parameter, which control system is insensitive to variations in the angular velocity and which yet has a relatively high stability. For this purpose the device comprises rotation speed detecting means for generating a clock signal having a frequency which is indicative of the rotation speed of the information carrier, the delay in the memory means being proportional to a period of the clock signal. In a device according to the invention the basis frequency is accurately matched to the rotation frequency. This makes it possible to reduce the frequency intervals of relatively high gain. On the one hand this provides for a good suppression of periodic disturbances, while on the other hand the control system can have a high stability.